feat(ai): support image-guided generation

[opencode-agent]

Summary

• add ordered images to the existing Image.generate({ model, prompt, images?, options?, http? }) API
• dispatch image-conditioned requests to OpenAI, xAI, and Gemini edit behavior while rejecting unsupported Z.ai image inputs before network I/O
• keep masks OpenAI-specific as typed options.mask?: ImageInput; only images is common across providers
• preserve provider-native open options, raw native field precedence, and documented http overlays
• add deterministic routing/type coverage plus live recorded edit cassettes with validated MIME and dimensions

Input model

ImageInput is a tagged union with constructors for MIME-bearing bytes, HTTP/data URLs, provider file IDs, and MIME-bearing provider file URIs. Raw strings are intentionally excluded to avoid ambiguous base64, URL, and provider-ID interpretation.

images is the only common image-editing input. OpenAI additionally accepts options: { mask: ImageInput.bytes(...) }; the adapter extracts that helper into the native multipart or JSON edit mask field rather than serializing the tagged helper object as a normal option.

Provider behavior

• OpenAI: /images/generations for text-only requests; /images/edits using multipart for bytes/data URLs and the documented JSON reference shape for URLs/file IDs. Multipart owns its content-type boundary and protects structural model, prompt, image[], and mask fields from raw body overlays.
• xAI: /images/generations without inputs; JSON /images/edits with singular image or ordered plural images; bytes become data URLs.
• Google: same generateContent route with ordered inlineData/fileData parts; public HTTP URLs fail before network I/O.
• Z.ai: image inputs fail before network I/O because the hosted international image API is text-only.
• OpenAI Responses hosted image-tool behavior is unchanged.

Verification

• package-scoped Prettier
• bun typecheck
• bun run build
• bun test --timeout 30000 (395 passed, 29 skipped)
• OpenAI, xAI, and Google edit cassette replays passed with cassette files unchanged
• changed-file secret scan passed

Requested by: @rekram1-node (Aiden Cline via Slack)